Allow GRPC_ENABLE_FORK_SUPPORT environment variable to be set to 'false'

[sreecha]

Currently we could only set the environment variable GRPC_ENABLE_FORK_SUPPORT to truthy values (see https://github.com/grpc/grpc/blob/v1.8.4/src/core/lib/support/fork.cc#L43)

There might be some scenarios where the user might want to explicitly disable fork support (see #13998 for example). To support such cases, we should allow the environment variable to also have falsey values

[sebbov]

What does GRPC_ENABLE_FORK_SUPPORT exactly do and what am I losing by disabling it as a work-around to googleapis/google-cloud-python#4992?

[srini100]

@sebbov GRPC_ENABLE_FORK_SUPPORT was added to support usage where fork is not followed by exec. A pthread_atfork() handler was added to cleanup exiting threads before fork and manage the FDs after fork. This change introduced the deadlock bug which users can run into when doing fork/exec. As a quick workaround, GRPC_ENABLE_FORK_SUPPORT when set to false doesn't invoke this new logic. At this point this is tribal knowledge. A fix will be coming soon.